1. Field of the Invention
The invention relates to a process for the prepartion of diphenylamine, which can be substituted by lower alkyl and/or lower alkoxy, by dehydrogenation of the corresponding N-cyclohexylidene-aniline in the presence of a rhodium-containing supported catalyst.
2. Description of the Related Art
A process in which imines, such as N-cyclohexylideneaniline and derivatives thereof, are used as starting substances and are dehydrogenated in the gas phase in the presence of supported catalysts based on nickel, platinum, palladium or copper-chromium is known for the preparation of diphenylamine and derivatives thereof from German Offenlegungsschrift 2,331,878. The disadvantages of this process are the inadequate selectivity of the catalysts and their low stability under load, so that the yield of diphenylamine decreases greatly after only a few hours.
Nickel-chromium mixed catalysts which contain aluminium, copper and alkaline earth metal sulphates and alkali metal sulphates as secondary constituents, and nickelchromium mixed catalysts with addition of manganese and aluminium are described in German Offenlegungsschrift 2,520,893. The catalysts described achieve very high service lives of up to 6000 hours only under a hydrogen atmosphere, but the conversion and the selectivity are unsatisfactory. The service lives are considerably shorter in a carrier stream of inert gas, and, depending on the temperature, either significant amounts of cyclohexylaniline or carbazole occur as a troublesome by-product, in addition to methyldiphenylamine, which is difficult to remove. Further disadvantages of the catalysts described are their low stability under load and their limited ease of regeneration, which restricts their value for industrial use.
EP 208,933 describes rhodium supported catalysts, the particular feature of which is pretreatment of the support material with chromium salts and manganese salts. These catalysts are described as being suitable contacts for gas phase hydrogenation of completely or partially hydrogenated, optionally substituted hydroxydiphenyls, the reaction being carried out in a hydrogen atmosphere, that is to say without a carrier stream of inert gas. The selectivities in the dehydrogenations of 2-cyclohexylidene/2-cyclohexenyl-cyclohexanone mixtures described in the examples do not, in general, exceed the 92% mark.
German Offenlegungsschrift 3,801,754 describes rhodium supported catalysts, the particular feature of which is, in addition to pretreatment of the support material with chromium salts and manganese salts, additional doping with at least one metal from the group comprising palladium, platinum and iridium. These catalysts are described as being suitable ones for gas phase dehydrogenation of optionally substituted dicyclohexylamines to give the corresponding diphenylamines, the presence of cyclohexylanilines and aniline and that of ammonia being described as not troublesome. Hydrogen, nitrogen and alkanes can be employed as carrier gases.
In German Offenlegungsschrift 3,801,754, the processes described hitherto for diphenylamine synthesis starting from partially hydrogenated, optionally substituted diphenylamines, that is to say also the dehydrogenation of cyclohexylidene-anilines, are described as being inadequate for industrial use.
The inadequacies of the dehydrogenation of cyclohexylidene-anilines are to be seen in association with the limited chemical stability of this class of compounds, which tend to undergo self-condensation, aniline being split off and oligomeric cyclohexenyl compounds being formed. The limitation of German Offenlegungsschrift 3,801,754 to the very stable dicyclohexylamines is to be assessed against this background. Since exclusively catalysts based on the support material claimed in EP 208,933 are described in German Offenlegungsschrift 3,801,754, with the difference that in addition to rhodium, another platinum metal is precipitated onto the support material, it follows that the catalysts from EP 208,933 should be of only limited suitability, that is to say when certain substrates are used, for diphenylamine synthesis by dehydrogenation.